1. Field of the Invention
This invention relates to a refastenable closure system for disposable garments, such as disposable diapers, and the method for preparing disposable garments with this improved closure system.
2. Background Information
Disposable diapers are generally comprised of a liquid-permeable inner skin-contacting layer, a liquid impermeable outer layer, and an absorbent layer disposed between these inner and outer layers. The inner layer can be a nonwoven or other soft liquid-permeable material. Nonwoven materials of polyolefin-based fibers are particularly useful as the inner layer. The absorbent layer can be formed of cellulosic fibers, polyolefin-based nonwoven webs which can contain absorbent particulate materials, or like materials. The outer layer of disposable diapers can be liquid-impermeable polyolefin-based films, preferably with a matte or embossed surface. The outer layer can be a polyolefin-based microporous film which is impermeable to liquids but permeable to gases, such as the polyolefin-based films described in U.S. Pat. Nos. 4,593,256 (Shipman), No. 4,609,584 (Cutler et al.), and No. 4,726,989 (Mrozinski).
A disposable diaper typically includes a closure system for securing the diaper about the wearer. This closure system typically comprises pressure-sensitive adhesive fastening tape tabs which are adhered to the marginal edges of the diaper, near one end, in the waist area of the diaper, in such a manner that a portion of each tab is free for use to adhere to the outer layer of the diaper at the opposite end to secure the diaper about the wearer. It is desirable that the closure formed by these pressure-sensitive adhesive fastening tape tabs be quite secure, to avoid unwanted opening of the closure. However, it is also desirable that the closure be capable of being repeatedly opened and reclosed, i.e., be refastenable, to allow for adjusting the fit of the diaper or for checking for a soiled condition.
The outer layer of a disposable diaper preferably comprises a polyolefin based film. In the interest of economy and to minimize stiffness of the diaper it is desirable that this outer layer be a thin film, typically in the range of 20 to 40 microns thick. As such, these films are of limited strength and this limited strength can be a limiting factor on how secure a closure can be formed and remain functionally refastenable. An aggressive pressure-sensitive adhesive fastening tape tab can distort or tear these thin film outer layers when the closure is opened, thus destroying the functionality of the closure system. To overcome this problem, various techniques have been proposed for reinforcing the outer layer in the area where the pressure-sensitive adhesive fastening tape tabs are adhered to secure the diaper about the wearer.
Various techniques are known in the art for reinforcing the outer impermeable film of disposable diapers. One technique involves coating the film with a reinforcing material either on the inner or outer surface of the film. U.S. Pat. No. 4,296,750 (Woon et al.) discloses coating the interior surface of the outer impermeable film with a hot-melt adhesive layer which has both a Ring and Ball softening point lower than that of the film and a modulus of elasticity lower than that of the film. U.S. Pat. No. 4,210,144 (Sarge III et al.) discloses a material having high tensile strength and a low elongation to tensile force property, relative to the outer impermeable film, coated on the outer impermeable film, preferably on the inner surface of the film, to both bond the film to the absorbent pad of the diaper and to reinforce the film. Problems with this technique include the cohesive strength of the coatings, which can be too low to allow lasting and complete holding of an adhesive fastening tape pressed against it; the heat of the hot-melt adhesives which can distort the thin polyolefin film during the coating process; and variations in the thickness of the coatings which can result in non-uniform reinforcement.
Another technique for reinforcing the outer layer of disposable diapers involves adhering a plastic strip to the area of the diaper to which the fastening tape is adhered to fasten the diaper around the wearer. U.S. Pat. No. 3,867,940 (Mesek et al.) discloses water-impervious backing sheets reinforced against stretching and rupture by having adhered to a minor portion of the surface thereof a flexible structural material having a higher modulus of elasticity than said backing sheet, wherein the flexible reinforcing structural material may be a scrim, such as a cotton gauze, or polyethylene filaments, or film material such as biaxially oriented polyethylene terephthalate, which can be bonded to the water-impermeable sheet by known methods, or if plyethylene gauze is used, by heat and pressure. European Patent Publication No. 0,080,647 (Boussac Saint Freres) discloses a plastic strip such as a smooth-surfaced polypropylene strip adhered to the impermeable film by a layer of adhesive. German Offenlegungsschrift No. 33 38 201 A 1 (Molnlycke) discloses a plastic strip of preferably polyethylene or polypropylene or polyester firmly adhered to the impermeable film. U.S. Pat. No. 4,753,649 (Pazdernik) discloses an article including an adhesive tape tab and a reinforced substrate in which the substrate is reinforced by bonding a reinforcing layer to the substrate wherein the reinforcing layer has a matte finish and a tensile strength of at least about 15 MPa and the reinforcing layer provides a peel force of at least about 11.7 N per inch of adhesive tab width when adhered to the adhesive tab.
In commercial practice, the above methods involve reinforcing a substrate, e.g. the outer layer of a disposable diaper, with a plastic strip, and the plastic strip is generally bonded to the substrate by one of two basic methods. One of these methods involves the reinforcing strip being provided from a roll of pressure-sensitive adhesive tape. While this method has the advantage of being a convenient method of bonding to the substrate, it also has a significant disadvantage in that to allow a uniform low-force unwinding of the adhesive-coated plastic reinforcing strip from a storage roll, the top surface of the reinforcing strip, i.e., the surface which becomes the target area to which the fastening tape is adhered in fastening the garment about the wearer, is typically coated with a release coating, thereby minimizing adhesion between the adhesive layer and the top surface. However, minimizing adhesion to the target area where the fastening tape is to be adhered is contrary to an intended characteristic of the target area, i.e., the characteristic of providing a surface to which the fastening tape can be reliably and lastingly adhered while still being capable of being peeled off of the target area without damage to the reinforcing layer or the reinforced substrate to allow for multiple closings and openings of the closure system.
The other method involves coating the plastic film with an adhesive on the manufacturing lines, for example diaper manufacturing lines, and using this adhesive to achieve the bond between the substrate and the reinforcing strip. This method does not require a release coating on the top surface of the reinforcing film as no such release coating is required to allow a roll of film to readily unwind. However, as with the methods which involve coating hot-melt adhesives onto the substrate there are several disadvantages associated with this method of achieving a reinforced substrate. These disadvantages include the complexity of such coating operations on high speed manufacturing lines, such as diaper manufacturing lines, difficulty in achieving a uniform coating, and difficulty in coating the adhesive to the edges of the reinforcing film.
One way of achieving a reinforced substrate closure system for disposable diapers which eliminates the use of a release coating while avoiding coating hot-melt adhesives on the manufacturing lines is found in U.S. Pat. No. 4,710,190 (Wood et al.) which discloses the use of a bilayer film as a reinforcing film for the outer layer of disposable diapers. The bilayer film comprises a reinforcing layer and a room-temperature-nontacky bonding layer, which is bonded to the outer layer of the diaper with heat and pressure. The reinforcing layer is free of release treatments on its top surface yet supply rolls of the bilayer film do not resist uniform low-force unwinding due to the nontacky nature of the bonding layer at room temperature. Although the use of such bilayer films overcomes the problems cited above it also suffers from the disadvantage of requiring the modification of diaper manufacturing lines to include a means for heating the bonding layer of the bilayer film, to affix pieces of this bilayer film to the substrate which forms the outer surface of the disposable diapers.
Another technique for reinforcing substrates is found in U.S. Pat. No. 4,643,730 (Chen et al.) which discloses coating at least a portion of the surface of the substrate with a layer of material that is curable by high energy radiation and curing the coating with a suitable source of high energy radiation to form a reinforced layer affixed to the substrate. The coating compositions disclosed include urethane acrylate oligomers, acrylated acrylic oligomers, and epoxy acrylate oligomers. The radiation sources disclosed include ultraviolet radiation sources and electron beam sources. The problems associated with this technique include the complexity of the method which can potentially reduce the efficiency of manufacturing lines, such as diaper manufacturing lines, the potential for non-uniform reinforcement as a result of non-uniform coatings, the relatively high cost of the materials, the potential for unpleasant odor of the uncured compositions due to low molecular weight components, and the potential for residual odor of the cured coatings due to incomplete cure.
Subjecting films to ionizing plasmas, such as produced by electrical discharge corona or flame, is well known in the art to provide an increase in the adhesion, to the film of various types of coatings which might be applied to the films as well as the adhesion of pressure-sensitive adhesive tapes. (See D. Briggs, Surface Treatment for Polyolefins, D. M. Brewis, Ed., MacMillan N.Y. 1982, Chapter 9, pp. 199-226.) A similar result has been reported for silicone release coatings when treated with a corona electrical discharge. U.S. Pat. No. 3,632,386 (Hurst) discloses that by subjecting a silicone polymer release surface to "oxidative" treatment, e.g., electrical discharge (corona) or flame treatment, the release properties of the surface are reduced, e.g., made more difficult. Hurst also discloses that the effect of this oxidative treatment, that of decreasing release properties of a silicone polymer release surface, is temporary unless the treated surface is protected after treatment, for example by coating the treated surface with an adhesive.